Weight Painting - using a single colour
I'm not sure of what you mean.
Weight painting needs at least two colors no matter what because it has to represent values between 0 and 1. So you will always have at least one color for 0, one color for 1 and it will display in between colors for every value between.

If you want to have only two colors for your weights paint (ie black and white), just set only two colors in your userprefs:



If you want to have as little transition as possible between values, use the "constant" interpolation and decay one of the sliders: